A local access network is a network that connects individual users, i.e., subscribers, to a central office (CO), either directly or through one or more host digital terminals and/or remote nodes. The CO is a switching node which is a part of a larger network. For example, in the historic telephony network, the CO was responsible for serving one or more telephone exchanges, i.e., groups of subscribers sharing the first three digits of a seven digit telephone number, and was a part of the larger telephony network. The larger telephony network comprised several COs connected to each other by interexchange trunks, and also connected to long-distance networks.
Historically, twisted-pair copper wires were used to connect a CO to individual users. However, widespread broadband access is anticipated in the near future, and copper wires have a limited capacity. Optical fiber, on the other hand, has excellent transmission characteristics, and a capacity that far exceeds that of copper wire. As a result, optical fiber is a preferred choice for new communication infrastructures. Because it is expensive to install and upgrade infrastructure, new infrastructure is preferably xe2x80x9cfuture-proof,xe2x80x9d i.e., able to support any conceivable service for the foreseeable future. The high capacity of optical fiber offers the potential of a future-proof infrastructure.
Optical fiber is also being deployed into local access networks with increasing frequency. Examples of local access network structures that incorporate fiber-optic cable can be found in N. Frigo, A Survey of Fiber Optics in Local Access Architectures, ch. 13 of Optical Fiber Communications, volume IIIA (Kaminow and Koch, eds., 1997). Existing local access network architectures incorporating optical fiber are usually similar to the architectures used for the older, copper wire networks. However, optical fibers are very different from copper wires, and these existing architectures may not be well suited for use with optical fibers. For example, optical fibers are typically congregated into fiber-optic cables containing many optical fibers and are capable of serving a far greater number of users than a copper cable of comparable size. As a result, a single cut in a fiber-optic cable could interrupt service to a larger group of users than a cut in a copper wire of similar size. An interruption of service to such a larger group of users from a single cable cut may be considered unacceptable. As a result, there is a need for a new architecture for local access networks that is more reliable than existing architectures.
FIG. 1 (prior art) shows a simplified schematic of an unprotected local access network 100 serving a cable group. As used herein, the term xe2x80x9ccable groupxe2x80x9d refers to all of the homes, apartments and offices served by one cable. A CO 110 is connected to the backbone of a communications network (not shown). A fiber-optic cable 120 comprises a plurality of optical fibers 140, each of which may be connected to CO 110. In particular, CO 110 may have one or more central office transceivers (COTs) 115; each optical fiber in fiber-optic cable 120 may be connected to a COT 115. At a Cable Access Point (CAP) 130, an optical fiber 140 is separated from fiber-optic cable 120, and is connected to a remote node (RN) 150. If optical fiber 140, as separated from fiber-optic cable 120, cannot reach RN 150, the length of optical fiber 140 may be increased by splicing an additional piece of optical fiber onto the end of optical fiber 140. This additional piece of optical fiber is considered a part of optical fiber 140. Such splicing may be performed generally and is not limited to the architecture of FIG. 1.
RN 150 is also connected by an optical fiber 160 to an optical network unit (ONU) 170. RN 150 may also be connected to a number of other ONUs similar to ONU 170, by optical fibers similar to optical fiber 160. RN 150 is adapted to split an optical signal from optical fiber 140 into N optical signals, one for each ONU connected to RN 150, and/or to combine N optical signals, one from each ONU connected to RN 150, into a single optical signal for optical fiber 140. For example, RN 150 may be a 1xc3x97N optical star coupler, adapted to split the optical signal from fiber 140 into N identical signals, one for each ONU connected to RN 150. RN 150 may also be a wavelength grating router (WGR), adapted to split the optical signal from optical fiber 140 into N possibly different signals, differentiated by wavelength, one for each ONU connected to RN 150. As used herein, xe2x80x9cONUxe2x80x9d refers to a terminal part of a local access network that provides an interface between the local access network and customer premises equipment (CPE), such as a telephone, facsimile machine, television, and/or computer. For example, an optical network unit may serve one or more houses, offices, or apartments. In the architecture of FIG. 1, a failure between a CAP 130 and ONUs 170 may leave N ONUs without service. However, a failure of fiber-optic cable 120 may leave the entire cable group without service.
FIG. 1 also shows two CAPs in addition to CAP 130, two RNs in addition to RN 150, each connected to three ONUs, such as ONU 170. These CAPs, RNs, and ONUs function in a manner similar to CAP 130, RN 150, and ONU 170. For clarity, FIG. 1 shows only three CAPs, including CAP 130, each having a single RN, such as RN 150, where each RN is connected to three ONUs, such as ONU 170. In reality, a local access architecture may have many more CAPs, RNs and ONUs. For example, in a typical telephony network, a single CO might be connected to on the order of hundreds of RNs 150 through a single fiber-optic cable, and each RN 150 might be connected to about 1-64 ONUs 170, or more likely 32-64 ONUs 170. These numbers are, of course, subject to wide variations, depending on the needs of the community served by the local access network.
Also for clarity, FIG. 1 shows ONU 170 connected to only a single RN 150. However, it is known to provide multiple connections from a single ONU to multiple RNs, for the purpose of providing multiple channels to each ONU. For example, one optical fiber could be used for transmissions to the ONU, and another could be used for transmissions from the ONU. Where such multiple connections are provided, each of optical fibers 140 and 160 as shown in FIG. 1 represent two or more optical fibers. FIG. 1 also shows only a single optical fiber 140 separated from fiber-optic cable 120 at CAP 130. However, it is known to separate a plurality of optical fibers at a CAP for the purpose of connecting to a plurality of RNs. Multiple RNs at a CAP could provide multiple connections to each ONU for the purpose of providing multiple channels, or could be connected to ONUs too numerous or too inconveniently located to be served by a single RN.
Large quantities of optical fiber have already been deployed in the backbones of telephone networks, which serve millions of users. It has long been realized that a cable cut in one of these backbones, which could affect hundreds of users, must not interrupt service for more than a moment. The high reliability required of the backbone is often achieved by using a Synchronous Optical NETwork (SONET) ring architecture.
FIG. 2 (prior art) shows a SONET ring architecture. SONET ring 200 has a plurality of COs 210, including CO 210a, CO 210b, CO 210c and CO 210d. Each CO has an add-drop multiplexer (ADM) 215. In particular, COs 210a, 210b, 210c and 210d have ADMs 215a, 215b, 215c and 215d, respectively. An ADM is a network element that can add and drop signals, such as SONET signals, from a line signal. Each CO 210 may also be connected to a local access architecture (not shown), and one or more of the COs 210 may also be connected to a larger communications network (not shown). ADM 215a is connected to ADM 215b by fiber-optic cables 220a and 230a. ADM 215b is connected to ADM 215c by fiber-optic cables 220b and 230b. ADM 215c is connected to ADM 215d by fiber-optic cables 220c and 230c. ADM 215d is connected to ADM 215a by fiber-optic cables 220d and 230d. If a cut occurs in any of the fiber-optic cables, there is at least one and possibly more alternate routes between the two ADMs connected by the failed fiber-optic cable. COs 210 are switching nodes, where the switching function is performed by ADMs 215. As such, COs 210 require power and significant maintenance.
A number of SONET ring architectures are described in Chapter 4 of Wu, Fiber Network Service Survivability (1992), which is incorporated by reference. Each of these architectures is similar to the architecture of FIG. 2, in that the SONET ring has a number of switching nodes, and alternate paths are provided between the switching nodes. If a cable cut occurs, data can be routed through an alternate path. Architectures similar to SONET ring architectures for a local access network are described by Chapter 8 of Wu, Fiber Network Service Survivability (1992), which is incorporated by reference. For example, Wu describes a number of architectures having a CO connected to a RN by a primary route and an alternate route, where the RN has an optical switch that chooses which route to use. While these architectures provide good reliability, they rely on switches distributed throughout the local access network in the RNs. It is very undesirable to have switches distributed throughout a local access network, because such switches require power and maintenance, and providing power and maintenance at decentralized locations significantly increases cost.
A ring architecture having switching functions consolidated to some degree at a single node is disclosed by Wagner et al., Multiwavelength Ring Networks for Switch Consolidation and Interconnection, IEEE International Conference on Communications, page 1173 (1992). However, this ring architecture has xe2x80x9cmany concatenated passive components,xe2x80x9d due to the bus architecture used by Wagner. As a result, the number of nodes that can be supported by the architecture is very low, on the order of 10, unless components that require power, such as amplifiers, are distributed throughout the network. Moreover, each node in the architecture has xe2x80x9celectronic selection,xe2x80x9d which also requires power. A local area network may require a number of nodes far greater than 10.
Most existing local access network architectures have neither the capacity nor the level of reliability provided by a SONET ring and similar architectures. Moreover, the economics of local access network architectures are very different from those of the backbone of a communication network, such that providing an architecture similar to that of SONET rings in the local access network is a very expensive proposition. In particular, SONET rings and similar architectures are based on several switching nodes that require power connected by fiber-optic cable, or nodes having some other functionality that requires power. In a local access network, providing power at nodes distributed throughout the network significantly raises cost. Moreover, a local access network may have many remote nodes. As a result, there is a need for a local access network architecture adapted to serve many nodes that has a high reliability, and does not require power at locations other than the CO and the ONUs. In addition, the expense of laying fiber is significant. There is therefore a further need for a high reliability local access network in which optical fiber is deployed in a cost-effective manner.
The present invention provides a local access network, having a switching node, a passive remote node connected to an optical network unit, a first optical fiber that provides a dedicated connection between the switching node and the passive remote node, and a second optical fiber that provides a dedicated connection between the switching node and the passive remote node. A first portion of a first fiber-optic cable containing the first optical fiber does not contain any part of the second optical fiber, such that there are independent paths from the switching node to the passive remote node.
The present invention further provides a local access network, having a switching node, a first passive remote node, a first optical fiber connecting the switching node to the first passive remote node, a second passive remote node, a second optical fiber connecting the switching node to the second passive remote node, an optical network unit, a third optical fiber connecting the first passive remote node to the optical network unit, and a fourth optical fiber connecting the second passive remote node to the optical network unit. A first portion of a first fiber-optic cable containing the first optical fiber does not contain any part of the second optical fiber, such that there are independent paths from the switching node to the optical network unit.